In general, fermentation products are produced by degradation of starch-containing material into fermentable sugars by liquefaction and saccharification followed by conversion of the sugars directly or indirectly into the desired fermentation product using a fermenting organism.
However, the industrial production of fermentation products such as ethanol and lactic acid is facing the challenge of redirecting the production process from fermentation of relatively easily convertible but expensive starchy materials, to the complex but inexpensive lignocellulosic biomass such as plant biomass.
Unlike starch, which contains homogenous and easily hydrolyzed polymers, lignocellulosic biomass contains variable amounts of cellulose, hemicellulose, lignin and small amounts of protein, pectin, wax and other organic compounds. Cellulosic biomass is a vast poorly exploited resource, and in some cases a waste problem. However, hexoses from cellulose can be converted by yeast to fuel ethanol for which there is a growing demand. Pentoses from hemicellulose cannot yet be converted to ethanol commercially but several promising ethanologenic microorganisms with the capacity to convert pentoses and hexoses are under development.
Typically, the first step in utilization of lignocellulosic biomass is a pre-treatment step, in order to fractionate the components of lignocellulosic material and increase their surface area. The pre-treatment method most often used is acid hydrolysis, where the lignocellulosic material is subjected to an acid such as sulphuric acid whereby the sugar polymers cellulose and hemicellulose are partly or completely hydrolysed to their constituent sugar monomers and the structure of the biomass is destroyed facilitating access of hydrolytic enzymes in subsequent processing steps. Another type of lignocellulose hydrolysis is steam explosion, a process comprising heating of the lignocellulosic material by steam injection to a temperature of 190-230° C. A further method is wet oxidation wherein the material is treated with oxygen at 150-185° C. The pre-treatments can be followed by enzymatic hydrolysis to complete the release of sugar monomers. This pre-treatment step results in the hydrolysis of cellulose into glucose while hemicellulose is partially or completely transformed into the pentoses xylose and arabinose and the hexoses glucose, mannose and galactose. Thus, in contrast to starch, the hydrolysis of lignocellulosic biomass results in the release of pentose sugars in addition to hexose sugars. This implies that useful fermenting organisms need to be able to convert both hexose and pentose sugars to desired fermentation products such as ethanol.
After the pre-treatment, the lignocellulosic biomass processing schemes involving enzymatic or microbial hydrolysis commonly involve five biologically mediated transformations: (1) the production of saccharolytic enzymes (cellulases and hemicellulases); (2) the hydrolysis of carbohydrate components present in pretreated biomass to sugars; (3) the fermentation of hexose sugars (e.g. glucose, mannose, and galactose); (4) the fermentation of pentose sugars (e.g., xylose and arabinose) and (5) the conversion of sugar alcohols like sorbitol, mannitol or xylitol.
Each processing step can make the overall process more costly and, therefore, decreases the economic feasibility of producing biofuel or carbon-based chemicals from cellulosic biological material. Thus, there is a need to develop methods that reduce the number of processing steps needed to convert cellulosic biological material to biofuel and other commercially desirable materials.
The five biologically mediated transformations may occur in a single step in a process configuration called consolidated bioprocessing (CBP), which is distinguished from other less highly integrated configurations in that CBP does not involve a dedicated process step for cellulase and/or hemicellulase production. CBP offers the potential for higher efficiency than processes requiring dedicated cellulase production.
Current CBP processes include extensive and costly pretreatment of the material by mechanical, thermochemical, and biochemical processes. Generally, the goals of such pretreatment processes include (1) rendering the cellulosic and hemicellulosic polymers more accessible to microorganisms, and (2) converting the complex cellulosic and hemicellulosic polysaccharides into simpler, fermentable sugars or other simple compounds, that are more readily converted into fuels and other chemicals by microorganisms. The mechanical, thermochemical, and biochemical processes frequently used in the pretreatment of lignocellulosic material constitute a major cost and are not completely effective.
Furthermore, the microorganisms currently used for the production of fuels and other chemicals from lignocellulosic material lack the necessary cellular machinery for both breaking down the complex plant polysaccharides into sugars (saccharification) and then converting the various resulting sugars into fuels and other chemical products in an efficient manner.
Ideally, desirable characteristics of different microorganisms could be utilized simultaneously by fermenting lignocellulosic biomass with co-cultures of the microorganisms. However, the optimal conditions for fermentation of lignocellulosic biomass vary greatly from species to species. Under the most favorable conditions, monocultures of bacteria can replicate very quickly and efficiently produce the desired fermentation product. However, due to evolutionary pressure, when a co-culture of microorganisms is present, the species that can grow the fastest often dominates. Many variables influence the success of bacterial fermentation of lignocellulosic biomass, including but not limited to: temperature, pH, growth medium, and pre-treatment protocol. Identifying the small window of conditions suitable for co-culturing at least two microorganisms, while the organisms simultaneously ferment lignocellulosic biomass, presents a significant challenge.
Thus, there remains a substantial unmet need for bioconversion processes that take advantage of better microorganisms and/or combinations of microorganisms in order to convert a broader spectrum of lignocellulosic biomass and saccharify complex polysaccharides to fermentable sugars for fermenting fuels and other chemicals.
Therefore, the availability of novel microorganisms and/or combinations of microorganisms for converting lignocellulosic biomass to high levels of carbon-based chemicals would be advantageous.